1. Field of the Invention
Proportioning devices for vehicle hydraulic brake systems.
2. Description of the Prior Art
In conventional proportioning valves for hydraulic vehicular brake systems, the effect of weight transferred from the rear wheels to the front wheels during a braking operation is compensated such that upon attainment of a selected level of output pressure from the master cylinder of the associated brake system, further pressure increases to the rear brakes occur at a preselected ratio, resulting in the brake pressure supplied to the rear brake system being predeterminately less than the master cylinder pressure, whereby to deter premature lock-up of the rear wheels as the weight of the vehicle is transferred from the rear wheels to the front wheels thereof.
Although proportioning valves heretofore known and used have been capable of closely approximating the idealized relationship between the pressure to the front brakes (often the master cylinder) and the pressure to the rear brakes, under varying load conditions, substantial deviations occur between the idealized relationship and the actual performance of the proportioning valves. The problem of modifying the operation of the proportioning valves in accordance with the vehicle loading has been approached in a number of different ways, such as through the use of connecting linkages between the proportioning valves and the associated vehicle suspensions. Additionally, the use of so-called "blend-back" valves have been used to improve the operation of conventional proportioning valves, one of which is shown in U.S. patent application Ser. No. 393,132, filed Aug. 30, 1973; however, it has been found that blend-back type valves work satisfactorily only if there exists a significant increase in the front brake pressure when moving from an unloaded to a loaded vehicle weight, and therefore in many applications, such blend-back type valves are not entirely satisfactory.
The invention disclosed in U.S. patent application Ser. No. 124,214, filed Mar. 15, 1971 now abandoned, overcomes a number of objectionable characteristics of the linkage-type load sensitive proportioning valves and is fully applicable in a number of areas wherein blend-back valves cannot be used, and the present invention is generally related to the valve assembly shown in the aforementioned application.